Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-MRAM. The conventional MTJ 10 typically resides on a substrate 12. A bottom contact 14 and top contact 22 may be used to drive current through the conventional MTJ 10. The conventional MTJ, uses conventional seed layer(s) (not shown), may include capping layers (not shown) and may include a conventional antiferromagnetic (AFM) layer (not shown). The conventional magnetic junction 10 includes a conventional reference layer 16, a conventional tunneling barrier layer 18, and a conventional free layer 20. Also shown is top contact 22. Conventional contacts 14 and 22 are used in driving the current in a current-perpendicular-to-plane (CPP) direction. Typically, the conventional reference layer 16 is closest to the substrate 12 of the layers 16, 18 and 20.
The conventional reference layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional reference layer 16 is fixed, or pinned, in a particular direction. Although depicted as a simple (single) layer, the conventional reference layer 16 may include multiple layers. For example, the conventional reference layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic.
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a SAF or other multilayer. Although shown as perpendicular-to-plane, the magnetization 21 of the conventional free layer 20 may be in plane. Thus, the reference layer 16 and free layer 20 may have their magnetizations 17 and 21, respectively oriented perpendicular to the plane of the layers.
Because of their potential for use in a variety of applications, research in magnetic memories is ongoing. For example, a lower write current may also be desired. Some of these goals may be achieved with dual magnetic junctions. Conventional dual magnetic junctions may include two reference layers, two tunneling barrier layers and a free layer. If the magnetic moments of the reference layers are in a dual state (antiparallel), then their contribution to the spin torque is additive. Thus, a lower write current may be achieved. Some dual magnetic junctions have a synthetic antiferromagnet (SAF) layer for one reference layer, a first nonmagnetic spacer layer, a free layer and a reference layer that can be aligned during writing. Such a dual magnetic junction requires the SAF for the reference layer because the magnetic field at the free layer due to other layers in the magnetic junction is desired to be at or near zero. This can be achieved with the SAF. However, this and other dual magnetic junctions may be difficult to because of their thickness. Other dual magnetic junctions have may have other drawbacks. Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.